ORIG_HEAD points to whatever the previous HEAD was, before the last git pull (or other branch-modifying operation).
I find it particularly useful after pulling, because you can see the changes that you just pulled by running git log ORIG_HEAD..HEAD or git diff ORIG_HEAD..HEAD. No need to copy-paste the hashes from git pull's Updating abc7def..1337420 output - ORIG_HEAD..HEAD is always the right range of commits!
idk how well-known this is, but I only found out about it recently, despite using Git for years now. (Though I mostly use Git via GUIs, where it's a lot more obvious which commits are being added by a pull, so I had less of a need to look for a generic solution to this.)
Absolutely incredible trick I didn't know
